The present invention relates generally to a stainless steel alloy bipolar plate that exhibits low electrical contact resistance in a fuel cell environment, and more particularly to an assembly made from such a bipolar plate with a coating to reduce the contact resistance between the plate and a diffusion layer or related current-carrying component placed against the plate.
In many fuel cell systems, hydrogen or a hydrogen-rich gas is supplied through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowpath to the cathode side of the fuel cell. An appropriate catalyst (for example, platinum) is typically disposed as a layer and used to facilitate hydrogen oxidation at the anode side and oxygen reduction at the cathode side. From this, electric current is produced with high temperature water vapor as a reaction byproduct. In one form of fuel cell, called the proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell, an electrolyte in the form of an ionomer membrane is assembled between the anode and cathode. This layered structure is commonly referred to as a membrane electrode assembly (MEA), and is further layered between diffusion layers that allow both gaseous reactant flow to and electric current flow from the MEA. The aforementioned catalyst layer may be disposed on or as part of the diffusion layer.
To increase electrical output, individual fuel cell units are stacked with electrically conductive bipolar plates disposed between the diffusion layer and anode electrode of one MEA and the diffusion layer and cathode electrode of an adjacent MEA. In such a configuration, the bipolar plates separating adjacently-stacked MEAs have opposing surfaces each of which include flow channels separated from one another by raised lands. The channels act as conduit to convey hydrogen and oxygen reactant streams to the respective anode and cathode of the MEA, while the lands, by virtue of their contact with the electrically conductive diffusion layer that is in turn in electrical communication with current produced at the catalyst sites, act as a transmission path for the electricity generated in the MEA. In this way, current is passed through the lands of the bipolar plate and the electrically-conductive diffusion layer. Typically, the bipolar plates are made from graphite or a metal in order to be an electrically conductive link between the MEA and an external electric circuit.
Bipolar plates made from graphite are resistant to corrosion, exhibit good electrical conductivity and low specific density. Nevertheless, graphite plates are permeable to hydrogen, which can lead to significant losses in performance and efficiency. Moreover, graphite is difficult to manufacture, resulting in plates that are expensive and thicker than their metal-based counterparts. Thus, in situations where cost of fuel cell manufacture is an important consideration, metal-based bipolar plates may be preferable to graphite. In addition to being relatively inexpensive, metal plates can be formed into thin members, having sheet thickness of less than 0.25 millimeters, for example.
Because the bipolar plate operates in a high temperature and corrosive environment, conventional metals, such as plain carbon steel, may not be suitable for certain applications (such as automotive applications) where long life (for example, about 10 years with 6000 hours of life) is required. During typical PEM fuel cell stack operation, the proton exchange membranes are at a temperature in the range of between about 75° C. and about 175° C., and at a pressure in the range of between about 100 kPa and 200 kPa absolute. Under such conditions, plates made from noble metals may be advantageous, as they have desirable corrosion-resistant properties. Unfortunately, they are very expensive, thereby limiting their viability in transportation-related and related cost-sensitive applications. Furthermore, some metals, such as nickel (at least when used alone) experiences severe corrosion inside a fuel cell environment. Thus, despite the fact that nickel my be appropriate for use in an alkaline media, its use in a PEM does not follow.
Steel alloys, which can be formed into very thin (for example, between 0.1 and 1.0 mm) sheets, are considerably less expensive, but may not exhibit adequate corrosion resistance. Stainless steel (for example, in the form of iron-chromium or related compounds), with its improved corrosion resistance through the formation of oxides on the plate surface, may also be used; however, the tendency of stainless steel bipolar plates to passivate to achieve this increase in corrosion resistance also increases the contact resistance between the plate and an adjacent diffusion layer.
Transition metal coatings, such as titanium nitride (TiN), have been proposed to avoid the buildup of high contact resistance on the surface of stainless steel bipolar plates. While initial electrical resistance may drop, the conditions encountered in a fuel cell over time (especially at elevated temperatures) cause the TiN to convert to titanium dioxide (TiO2), which actually increases the contact resistance, thereby achieving a result opposite of that intended. As such, there remains a desire to provide stainless steel alloy-based bipolar plates that exhibit corrosion resistance and low contact resistance for use in a fuel cell. There is also a desire to make a bipolar plate that exhibits similar corrosion and contact resistance properties to those of noble metal-based or graphite-based bipolar plates while avoiding their high cost of manufacture.